General system specs for large scenes in blender
Blender's website has a page dedicated to this, it is now dated a few years but is still useful (was updated for 2.6).

http://archive.blender.org/features-gallery/requirements/index.html

If you are considering building a PC for 3D, anything from Average and up should suffice for fairly complex scenes.


  Minimal specs for Hardware
  
  
  32 bits, Dual Core CPU with at least 2 GHZ, and SSE2 support.
  2 GB RAM
  24 bits 1280x768 display
  Mouse or trackpad
  OpenGL Graphics Card with 256 MB RAM
  
  
  Average specs for Hardware
  
  
  64 bits, Quad Core CPU
  8 GB RAM
  Full HD Display with 24 bit color
  Mouse or trackpad
  OpenGL Graphics Card with 1 GB RAM
  
  
  Production specs for Hardware
  
  
  64 bits, Dual 8 Core CPU
  16 GB RAM
  Two times Full HD Display with 24 bit color
  Mouse + tablet
  Dual OpenGL Graphics Cards, quality brand with 3 GB RAM
  


Also related: What are the best graphics cards for cycles? and How important is a GPU for modeling?